UPD17P136ACT datasheet(56/292 Pages) NEC

56 / 292 page

CHAPTER 6 STACK

6.5 STACK POINTER (SP) AND INTERRUPT STACK REGISTERS

The stack pointer is a 3-bit binary counter that specifies the addresses of the five address stack registers as shown

in Figure 6-1, and is assigned to address 01H of the register file. The value of the stack pointer is initialized to 5H

at reset.

•

The value of SP is decremented by one when the CALL addr or CALL @AR instruction is executed, when the

first instruction cycle of the “MOVT DBF, @AR” instruction is executed, or when an interrupt is accepted.

•

The value of SP is incremented by one when the RET or RETSK instruction is executed, when the second

instruction cycle of the “MOVT DBF, @AR” instruction is executed, when the POP AR instruction is executed,

or when the RETI instruction is executed.

When an interrupt is accepted, the counter of the interrupt stack registers is also decremented by one in addition

to the SP. The value of the counter of the interrupt stack registers is incremented by one only when the RETI instruction

is executed.

Table 6-1. Operation of Stack Pointer

Instruction

Value of stack pointer (SP)

Counter of interrupt stack registers

CALL addr

CALL @AR

–1

MOVT, DBF @AR (1st instruction cycle)

PUSH AR

Not affected

RET

RETSK

MOVT DBF, @AR (2nd instruction cycle)

POP AR

Accepting interrupt

–1

RETI

Remark Two instruction cycles are required to execute the “MOVT DBF, @AR” instruction, but this is an

exception.

Because the stack pointer (SP) is a 3-bit binary counter, it can take a value 0H to 7H. If the value of the stack pointer

is 6 or more, however, an internal reset signal is generated (to prevent a hang-up). This is because only five address

stack registers are available.

Because the stack pointer is located on the register file, its value can be directly read by manipulating the register

file with the POKE instruction. The value of the stack pointer is also changed at this time, but the values of the address

stack registers are not affected. Of course, the stack pointer can also be read by using the PEEK instruction.

The value of the stack pointer is 5H at reset.

UPD17P136ACT Datasheet(PDF) 56 Page - NEC